Embodiments of the present invention generally relate to seals and more particularly, to honeycomb seals for turbo machines.
A compressor is a machine which accelerates gas particles to ultimately increase the pressure of a compressible fluid, for example, a process gas, through the use of mechanical energy. Compressors are commonly used in the energy industry to produce, process, re-inject and transport many different types of gases. Among the various types of compressors are the so-called centrifugal compressors, in which a rotor mounted impeller imparts a centrifugal acceleration to a process gas. More generally, centrifugal compressors can be said to be part of a class of machinery known as “turbo machines” or “turbo rotating machines”.
High speed rotating centrifugal compressors may be prone to rotor-dynamic instability. The compressor seals are the major source of destabilizing forces responsible for instability. This is particularly true in applications involving high pressure high density gas, such as natural gas reinjection. Consequently, a seal or seals may be responsible for preventing full speed, full load compressor operation. In more severe cases, rotor destabilizing forces introduced to the rotating assembly by a seal or seals may cause a catastrophic failure necessitating costly shut down and repair.
Some seals, such as honeycomb seals, are known for providing not only a relatively low maintenance sealing solution, but also for providing a damping effect which may counteract such destabilizing forces. Honeycomb seals are thus often implemented in centrifugal compressor applications to enhance rotor-dynamic stability.
As shown in FIG. 11, a honeycomb seal 514 may include a seal body 516 having a plurality of honeycomb cells 518. During operation, the honeycomb cells resist the flow of fluid through a seal gap between the seal body and the rotor to provide both a sealing function and a rotor stabilizing function. Unfortunately, under certain circumstances and in certain configurations, it has been found that the honeycomb seal may actually add an undesired negative stiffness and a destabilizing effect (cross coupled stiffness) to the rotor assembly. One solution proposed for addressing this effect has been to add a groove, such as groove 522 in FIG. 11, to “break up” the honeycomb seal and thereby inhibit the destabilizing direct negative stiffness effect. For a further discussion of the addition of a groove in a honeycomb seal, see for example, Childs et al., A Design to Increase the Static Stiffness of Hole Pattern Stator Gas Seals, ASME Turbo Expo 2006; Power for Land, Sea and Air May 8-11, 2006, GT2006-90778, the entirety of which is herein incorporated by reference.
It is also generally understood that fluid swirl introduced to process fluids by the rotation of the rotor shaft may play a role in the onset of cross coupling. To inhibit swirl, it has been proposed to add a vane or vanes to the compressor casing to direct process fluid opposite to the direction of swirl. It has also been proposed to add another seal to the rotor shaft to reduce swirl, see for example U.S. Pat. No. 5,540,447, issued on Jul. 30, 1996 to Shultz et al., the entirety of which is herein incorporated by reference.
The addition of a vane or vanes and/or yet another extra seal to the turbo rotor shaft results in further complexity and additional rotating mass, both of which are undesirable in the drive to improve compressor performance. What is needed is a seal for a turbo machine capable of providing improved sealing, improved stabilization, reduced cross coupling, and improved turbo machine performance.